This application is a U.S. National Phase Application of PCT International Application PCT/JP98/02428.
The present invention relates to a dual band switch, a dual band antenna duplexer and a dual band mobile communication apparatus using the same, used mainly for a mobile communication device such as a portable telephone or the like.
A popular convention high frequency switch is disclosed in the non-examined Japanese Patent Application Publication No. H07-321692. A conventional switch is shown in FIG. 13. The switch of FIG. 13 comprises a circuit connecting in parallel PIN diode 1001 and compensation circuit 1002. Compensation circuit 1002 may be formed with a series connection of capacitor 1003 and inductor 1004. Compensation circuit 1002 may be used for turning off the switch circuit when PIN diode 1001 is in an inactive state. Therefore, compensation circuit 1002 may be set so that inductor 1004 cancels the parasitic capacitance of PIN diode 1001 in an inactive state, and may help create parallel resonance at a desired band. Capacitor 1003 may be referred to as a DC cut element for interrupting the direct current route of the compensation circuit when PIN diode 1001 becomes active and the switch circuit is turned on. As a result, compensation circuit 1002 may be adjusted to have an impedance which is capacitive in a frequency range close to a direct current and inductive in a desired band, as well as to have one series resonance point in-between.
In recent years, the rapid increase of users of mobile communication technology has been observed. Mobile communication technology often entails obtaining a required number of telephonic communication channels. Experiments in using two band systems by one communication apparatus may therefore often be performed. For two band systems, a switch that works in two different bands may be required. A conventional high frequency switch, however, may be able to obtain a sufficient OFF state in only one band when a PIN diode is inactive. Therefore, to realize two band systems, two high frequency switches suitable for respective bands may be needed. The use of two high frequency switches may result in a large and complicated circuit, as well as a relatively expensive one.
The present invention addresses the aforementioned and other problems and aims to provide a dual band switch with which sufficient OFF states may be obtained in two different bands.
A circuit in accordance with the present invention comprises a parallel circuit including a diode and a compensation circuit. The compensation circuit is formed with a circuit having at least two series resonance points and one parallel resonance point.
A circuit in accordance with the present invention allows the impedance of the compensation circuit, which is capacitive in low frequency close to a direct current, to become inductive after the first series resonance point, whereby a parasitic capacitance of the diode is canceled in a first band. Further, the impedance of the compensation circuit becomes inductive again after the parallel resonance point and after the following series resonance point, whereby a parasitic capacitance of the diode is canceled in a second band. A dual band switch which assures sufficient OFF states in two different bands may thus be obtained with a relatively simple structure.